Conversion of hydrocarbon oils



Feb. 9, 1937., H AN ELL 2,070,249

CONVERSION OF HYDROCARBON OILS Filed Dec. 30, 1953 RECEIVER INVENTOR- CHARLES H NGELL ATTORNEY Patented Feb. 9, 1937 stare Application December 36, 1933, Serial No. 'liitfiilt 'e Claims.

This invention particularly refers to an imprc-ved process for the simultaneous topping, cracking and reforming of hydrocarbon oils of relatively wide boiling range such as crude potroleum or fractions thereof containing an appreciable quantity of motor fuel or motor fuel fractions of poor anti-knock value.

am aware that other processes have been proposed for the simultaneous topping, cracking and reforming of gasoline-containing crude oils wherein the straight-run gasoline is subjected to reforming under conversion conditions of elevated temperature and superatmospheric pressure for the purpose of improving its anti-knock value while the topped crude is simultaneously subjected to conversion under independently controlled heating conditions and wherein intermediate conversion products of the cracking and reforming operations are subjected to further conversion in the same system by returning them either to the same heating coil to which the topped crude is supplied or to the heating coll wherein the straight-run gasoline is subjected to reforming.

The present invention differs from previous operations of the general character above described, in that the topped crude is separated, together with the reflux condensate comprising insufficiently converted intermediate products of the cracking and reforming operations, into low-boiling and high-boiling fractions, the low boiling fractions being subjected to conversion under independently controlled conditions of elevated temperature and superatmospheric pressure in a heating coil separate from that wherein the motor fuel or motor fuel fractions resulting from the topping operation are subjected to reforming while the higher boiling fractions of the reflux condensate. and topped charging stock are sub-=- jected to relatively mild conversion conditions in the same system without passing them through a heating coil.

The advantages of the invention over the previous methods of operation above outlined reside in the cooperative relation and coordination of the various steps of the process whereby the total components of the charging stock, (excluding only any motor fuel fractions of good antiknock value) as well as intermediate conversion products of theprocess, are converted to produce high yields of motor fuel of good anti-kneels value without the excessive production of coke and gas. This is accomplished in the present invention by separating both the charging stock and the inter mediate conversion products into selected fractions, each of which is subjected to independently (El. MSW- i controlled conversion conditions most suitable for the production of good quality motor fuel; excess heat remaining in the products resulting from the desired conversion of the low-boiling oils being utilized to effect relatively mild conversion of the hlgh-b0lling fractions of the reflux condensate and topped charging stock without passing the high-boiling oils through a heating coil. By commingling said high-boiling fractions with the highly heated lower boiling fractions, after passage of the latter through a heating coil, more severe conversion conditions may be employed in the heating coil for the treatment of said lowboiling fractions than in an operation wherein high-boiling fractions are passed through the same heating coil, without excessive coke and gas formation. At the same time milder conversion of the high-boiling fractions may be accomplished, as compared with an operation wherein they are passed through a heating coil with the lower boiling fractions, but due to the cooperative nature of the features of the present invention such relatively mild conversion of the high-boiling oils does not cause a reduction in the final yield of good quality motor fuel, since, after separation of the desired residual conversion products, the insufiiciently converted intermediate products are separated into low-boiling and high-boiling fractions and subjected to further conversion in the same system. The cumulative result of the continuous segregation and selective conversion of the intermediate products as well as the charging stock results in a final maximum yield of the desired light liquid product with the controlled production of a minor quantity of residue of the desired characteristics and a minimum production of gas.

One specific embodiment of the invention comprises subjecting a hydrocarbon oil of relatively wide boiling range, containing an appreciable quantity of motor fuel or motor fuel fractions, to a topping operation whereby the motor fuel components are separated from the higher boiling components comprising the topped charging stock, subjecting the total motor fuel components or only their high-boiling fractions of inferior antiknock value to conversion conditions of elevated temperature and superatmospheric pressure in a heating coil, for the purpose of materially improving their anti-knock value without excessively altering their boiling range, separating the topped charging stools into relatively low-boiling and relatively high-boiling fractions, subjecting the low-boiling fractions to independently controlled conversion conditions of elevated term perature and superatmospheric pressure in a separate heating coil and a communicating enlarged reaction chamber, separating the vaporous and residual conversion products from both heating coils, subjecting the vapors to fractionation whereby their insufiiciently converted components are condensed as reflux condensate, separating the reflux condensate into low-boiling and high-boiling fractions, returning the former to further conversion in said separate heating coil, together with the low-boiling fractions of the topped charging stock, subjecting fractionated vapors of the desired end-boiling point to condensation, collecting and separating the resulting distillate and gas and commingling said highboiling fractions of the reflux condensate and said high-boiling fractions of the topped charging stock with the stream of heated oil passing from said separate heating coil into the reaction chamber. The highly heated motor fuel or motor fuel fractions from the first mentioned heating coil may be introduced into the same reaction chamber to which the heated oil from the second mentioned heating coil is supplied or may be commingled with the vaporous conversion products of the process at any desired subsequent point in the system, prior to fractionation of the vapors.

As a modification of the specific embodiment above described, which is also within the scope of the invention, the high-boiling fractions of the topped charging stock and reflux condensate may, when desired, be commingled with the highly heated motor fuel or motor fuel fractions from the topping operation, subsequent to the passage of the latter through the heating coil.

It is also within the scope of the invention to subject the residual liquid products resulting from the conversion of the various oils at high pressure to further vaporization at substantially reduced pressure, in which case a regulated quantity or all of the highly heated motor fuel or motor fuel fractions may, when desired, be introduced into direct contact with the residual material in the reduced pressure vaporizing zone for the purpose of assisting its vaporization which, when desired, may be carried to the point of reduction of the residual liquid to coke.

The invention may be utilized in a wide variety of different types of apparatus and it is not intended to limit the invention to the particular type of apparatus illustrated in the accompanying diagrammatic drawing, which is merely intended to illustrate one of the many specific forms of apparatus in which the invention may be carried out.

'Raw oil charging stock for the process, comprising crude petroleum or any other hydrocarbon oil of relatively Wide boiling range containing an appreciable quantity of motor fuel or motor fuel fractions, all or a substantial proportion of which are of inferior anti-knock value, is supplied through line I and valve 2 into topping column 3, being heated, prior to its introduction into this zone, to a temperature sufiicient to cause its substantial vaporization. Such heating may be accomplished by any well known means, not shown in the drawing, for example, by means of indirect contact and heat exchange with hot liquid and/or vaporous conversion products of the process, by means of heat recovered fromthe furnace flue gases, by means of heat from any suitable external source, or by any desired com bination of such means. The topping operation is preferably accomplished at substantially atmospheric pressure, although heating of the charging stock may be accomplished at substantial superatmospheric pressure, in which case either substantially the same or somewhat reduced pressure may be employed in the topping column.

Topping column 3 comprises a vaporizing and fractionating zone wherein the high-boiling fractions of the charging stock,.hereafter designated as topped charging stock, are separated from its lower boiling fractions comprising materials boiling within substantially the range of gasoline.

When the charging stock contains no low-boiling fractions of satisfactory anti-knock value, fractionation is controlled in column 3 by any well known method, one of which will be later more fully described, so that its total components boiling up to approximately 400 to 450 F., are removed as the overhead vaporous product from column 3, passing through line 4 and valve 5 to cooling and condensation in condenser E. The

resulting distillate and gas passes through line i and valve 8 to collection and separation in receiver 9. Uncondensable gas may be released from the receiver through line iii and valve II and the distillate is withdrawn through line H and valve i3 to pump IQ.

When an appreciable quantity of low-boiling motor fuel fractions of satisfactory anti-knock value are contained in the charging stock, fractionation in column 3 is preferably so controlled that these materials comprise the only distillate collected in receiver 9, in which case this material which may have an end-boiling point, for example, of from to 300 F., or thereabouts, is withdrawn from the receiver through line I5 and valve It to storage or elsewhere, as desired.

In either case a regulated portion of the dis tillate collected in receiver 9 may, when desired, be recirculated to the upper portion of column 3 to serve as a refluxing and cooling medium, assisting fractionation of the vapors and maintaining the desired vapor outlet temperature from the fractionator, thus controlling the end-bolling point of the overhead distillate product.

In case only low-boiling fractions of good anti-knock value are collected in receiver 9, the higher boiling motor fuel components of the charging stock of inferior anti-knock value, including, when desired, some materials within the range of naphtha or kerosene, may be separately removed from the fractionator at one or a plurality of suitable intermediate points, for example, through line H and valve 18, to stripping column !9 wherein this material may be substantially freed of any components within the range of the overhead vaporous products from column 3 and entrained uncondensable gas. A suitable heating medium such as steam or hot vaporous or liquid products from within the system may be circulated through a closed coil 20 in the lower portion of column IQ for the purpose of reboiling the condensate supplied to this zone. vaporous products evolved in column I 9 may be returned to column 3 through line 2i and valve 22 and the stabilized condensate is withdrawn from the lower portion of column 59 through line 23 and valve 24 to pump I l. I t will be understood that stripping column is is not an essential featureof the present invention and may be eliminated, when desired.

The light distillate supplied to pump I i, comprising motor fuel or motor fuel fractions of inferior anti-knock value and including, when desired, components of the charging stock boiling for example, up to approximately 500 F., is fed therefrom through line 25 and valve 26 to heating coil 21 wherein it is heated to the desired conversion temperature, preferably at a substantial superatmospheric pressure, by means of heat supplied from a furnace 28 of any suitable form. Conditions are controlled in heating coil 21 to effect an appreciable improvement in the motor fuel characteristics of the distillate supplied to this zone, particularly with respect to its antiknock value, without excessively altering its boiling range and the heated products are discharged therefrom through line 29 and may pass, for example, through valve 30 into reaction chamber 34 or may be discharged at any suitable point into subsequent portions of the cracking system, as will be later more fully described.

Chamber 3| is preferably maintained at a substantial superatmospheric pressure and, although not illustrated in the drawing, is preferably well insulated to conserve heat so that conversion of the heated oils supplied to this zone, and more particularly their vaporous components, may continue therein. Both vaporous and liquid conversion products are withdrawn, in the particular case illustrated, from the lower portion of chamber 3! through line 32 and valve 33 to vaporizing or coking chamber 34, although it is within the scope of the inventionv to withdraw only the liquid or the liquid and a minor portion of the vaporous conversion products in this manner, in which case all or the remaining major portion of. the vapors are separately withdrawn from any desired point in chamber 31, above the point of removal of the liquid conversion products, and may be directed by well known means, not shown in the drawing, either into chamber 34 or direct to fractionator 39 or, in part, to both of these zones.

Chamber 34 is preferably maintained at a substantially reduced pressure relative to that employed in chamber 3 I, by means of which further-vaporization of the liquid conversion products supplied to this zone is accomplished; the

reduction in pressure also serving to cool and retard or arrest appreciable further conversion of the-material supplied to chamber 34. Residual liquid remaining unvaporized in chamber 34 may be withdrawn therefrom through line 35 and valve 36 to cooling and storage or to any desired further treatment or, when desired, the process may be operated for the production of substantially dry coke in chamber 34, in which case the coke may be allowed to accumulate in this zone to be removed therefrom after the operation of the chamber has been completed. When desired, a plurality of coking chambers similar to chamber 34, but not illustrated in the drawing, may be employed and preferably are alternately operated, cleaned and prepared for further operation in order that the duration of the operation of the process will not be limited by the capacity of the coking zone, although simultaneous operation of a plurality of coking chambers is permissible Within the scope of the present invention. It is also within the scope of the present invention to operate chamber 34 for the production of liquid residue and to remove this product therefrom to one or a plurality of separate chambers, not illustrated, wherein it may be reduced to coke in any well known manner.

vaporous conversion products pass from chamber 39 through line 31 and valve 39 to fractionapreviously described, but not illustrated. The insufliciently converted components of the vaporous products supplied to fractionator 39 are condensed in this zone as reflux condensate and the reflux condensate is separated, as will be later more fully described, into low-boiling and highboiling fractions. Fractionated vapors of the desired end-boiling point are withdrawn from the upper portion of fractionator 39 through line 40 and valve 4| and are subjected to condensation and cooling in condenser 42, from which the resulting distillate and gas passes through line 43 and valve 44 to collection and separation in receiver 45. Uncondensable gas may be released from the receiver through line 46 and valve 41. The distillate may be withdrawn from this zone through line 48 and valve 49 to storage or to any desired further treatment. This distillate preferably comprises conversion products of the process boiling within the range of gasoline and of good anti-knock value and, when desired, may be blended with any low-boiling motor'fuel frac: tions of satisfactory anti-knock value collected, as previously described, in receiver 9, the blended material comprising, in this case, the final motor fuel product of the process. When desired, a regulated portion of the distillate collected in receiver 45 may be recirculated by well known means, not shown in the drawing, to the upper portion of fractionator 39 to assist cooling and fractionation of the vapors in this zone and to maintain the desired vapor outlet temperature therefrom, thus controlling the end-boiling point of the overhead product from this zone.

Simultaneous with the operation above described the topped charging stock, comprising its components boiling above substantially 400 to 500 F., or thereabouts, are withdrawn as condensate from the lower portion of topping column 3 through line 50 and valve 51 to pump 52 by means of which they are fed through line 53 and valve 54 and are introduced in fractionator 39 into direct contact with the vaporous conversion products of the process undergoing fractionation. The topped charging stock thus supplied to fractionator 39 serves to assist fractionation of the vaporous conversion products and by virtue of its direct contact and heat exchange therewith is subjected to vaporization and to the same fractionation as that afforded the vaporous conversion products with which it commingles. Components of the vaporous conversion products boiling above the range of the desired final light distillate product of the process are condensed as reflux condensate and separated, as previously indicated, into low-boiling and high-boiling fractions, the topped charging stock commingled therewith being similarly separated into low-boiling and high-boiling fractions corresponding, to the low and high-boiling fractions-of the reflux condensate.

Low-boiling fractions of the reflux condensate and topped charging stock may be withdrawn from one or a plurality of suitable intermediate points in the fractionator, for example, through line 55 and valve 56, to stripping column 51 wherein they may be substantially freed of any low-boilingcomponents within the range of the desired overhead vaporous product from fractionator 39 and entrained uncondensable gas. A heating coil 58 within the lower portion of column 51, through which\ steam or other suitable heating medium may be circulated serves as a means of reboiling the condensate supplied to this zone. Vaporous products evolved in column 51 may be returned to fractionator 39 through line 59 and valve 60. The reboiled and stabilized condensate may be withdrawn from the lower portion of columns 51 through line 5i and valve 62 to pump 63. Column 51, like column i9, is not an essential feature of the invention and may be eliminated, when desired. The low-boiling fractions of the reflux condensate and topped charging stock supplied to pump 63 are fed therefrom through line 66 and valve 05 to heating coil 66, wherein they are subjected to the desired conversion temperature, preferably at substantial superatmospheric pressure by means of heat supplied from a furnace El of any suitable form. The heated oil is discharged from heating coil 0% through line 68, valve 69, and line 70 into reaction chamber 3!.

The high-boiling fractions of the reflux condensate and topped charging stock are withdrawn from the lower portion of fractionator 39 through line H and valve E2 to pump 73 by means of which they are fed through line la and may pass through valve F5, in this line, into line l0,

' commingling therein with the heated products from heating coil 6% and passing therewith into reaction chamber 3|, wherein they are subjected to the conversion conditions prevailing in this zone, or the high-boiling fractions of the reflux condensate and topped charging stock may be diverted, all or in part, from line Hi through line it and valve ll into line 29, commingling therein with the heated products from heating coil 27 and passing therewith to chamber 3!. It is also within the scope of the present invention, instead of commingling the high-boiling fractions of the reflux condensate and topped charging stock with the heated products from either heating coil, to separately introduce this material into chamber St at any desired point by well known means, not shown in the drawing.

When desired, in order to assist vaporization of the residual liquid supplied to chamber 3%, a regulated portion or all of the highly heated dis tillate from heating coil 2i may be diverted from line 29 through line iii and valve '50 into chamber 36 and preferably into direct contact with the residual material in this zone. The additional heating thus afforded the residue may, when desired, be sufiicient to efiect its reduction to substantially dry coke. Introduction of the heated products from heating coil 21 into chamber 34 for the purpose mentioned may be accomplished without sacrificing the desired degree of conversion of the light distillate supplied to heating coil 27, since the low-boiling nature of the distillate supplied to this zone permits its conversion in the heating coil without the necessity of a subsequent high pressure reaction chamber for separation of any liquid conversion products and continued conversion of the vapors. When the heated oil from heating coil 21 is supplied to chamber 3| it serves to impart additional heat to the vaporous products in this zone and assist their continued conversion. When this is not required or desirable and when further vaporization of the residue in chamber 34, by the introduction of heated oil. from heating coil 21, is not required, the products from heating coil 27 may be supplied to the upper portion of chamber 35 or direct to fractionator 39, by well known means not shown in the drawing, and, when desired,

may be previously cooled by any well known means not shown, for example, by indirect 0on tact and heat exchange with charging stock for the process or any other cooler material either aovaaae from within the system or from an external source.-

In the operation of a process such as illustrated and above described, the preferred range of operating condition's'may be approximately as follows: A conversion temperature within the range of 850 to 975? F., or thereabouts, may be employed at the, outlet from .the heating coil to which the low-boiling fractions of the reflux condensate and topped charging stock are supplied, preferably with a substantial superatmospheric pressure of the order of to 500 pounds or thereabouts, per square inch, at this point in the system. A somewhat higher conversion temperature of the order of 950 to 1050 F., and preferably a super-atmospheric pressure of from 400 to 1000 pounds, or thereabouts, per square inch is preferred at the outlet from the reforming coil to which distillate from the topping operation is supplied. A substantial superatmospheric pressure within the range of 100 to 500 pounds, or thereabouts, per square inch, preferably substantially equalized with or, when desired, somewhat reduced relative to the pressure at the outlet from the heating coil employing the lowest pressure, in case both heating coils discharge into the reaction chamber, may be utilized in the reactionchamber and a substantially reduced pressure ranging, for example, from 100 pounds, or thereabouts, per square inch to substantially atmospheric pressure is preferred in the vaporizing or coking chamber. The pressure employed in the vaporizing or coking chamber may be either substantially equalized or somewhat reduced in the succeeding fractionating, condensing and collecting portions of the cracking system. As previously mentioned, the topping column is preferably operated at substantially atmospheric pressure although a superatmospheric pressure up to pounds, or thereabouts, may be employed in this zone, when desired, with substantially equalized or reduced pressure in the succeeding condensing and collecting equipment.

As a specific example of one of the many possible operations of the process, a mixed base California crude containing approximately 27% of 400 F., end-point straight-run gasoline is the charging stock for the process. This material is subjected to heating by means of heat recovered from within the system, to a temperature of approximately 550 F., at a superatmospheric pressure of about 150 pounds per square inch,

' and is introduced into the topping column where- 'the remaining light fractions of the crude, boiling up to approximately 450 F., are separately recovered as the side stream from the topping column and subjected, in the reformingcoil, to an outlet temperature of approximately 985 F., at a superatmospheric pressure of about 800 pounds per square inch, the topped crude is supplied to the fractionator of the cracking system wherein it is separated into high-boiling and low-boiling fractions, together with the reflux condensate resulting from fractionation of the vaporous conversion products of the process. The boiling range of said low-boiling fractions is approximately 400 to 625 F., and this material is subjected in a separate heating coil to an outlet conversion temperature of approximately 950 F., at a superatmospheric pressure of about 350 pounds per square inch. In this operation the products from both heating coils are introduced into the reaction chamber and the high-boiling fractions of the reflux condensate and topped charging stock are coimningled therewith and subjected to the conversion conditions prevailing in the reaction chamber which is maintained at a sup'eratmospheric pressure of about 350 pounds per square inch. A reduced pressure of approximately 75 pounds per square inch is employed in the vaporizing chamber and is substantially equalized in the succeeding fractionating, condensing and collecting portions of temperature is employed at the outlet of the Cir heating coil to which the low-boiling fractions of the reflux condensate and topped charging stock are supplied and the highly heated distillate from the reforming coil is introduced into direct contact with the residue in the coking chamber which is operated at a slight superatmospheric pressure (about 10 pounds per square inch). About 80 pounds of low volatile coke will be obtained per barrel of charging stock with a slight increase in the yield of motor fuel and gas.

I claim as my invention'.

1. A process for the treatment of hydrocarbon oils which comprises subjecting raw oil charging stock for the process to a topping operation whereby its light fractions, boiling within substantially the range of motor fuel, are separated from its higher boiling components, comprising topped charging stock, separating the latter into low-boiling and high-boiling fractions, subjecting the low-boiling fractions to conversion temperature at superatmospheric pressure in a heating coil and communicating reaction chamber, separating the resulting vaporous and residual conversion products, subjecting the vapors to frac-.

tionation whereby their insufliciently converted components are condensed as reflux condensate, subjecting fractionated vapors of the desired end-boiling point to condensation, separating the reflux condensate into low-boiling and'high-boiling fractions, returning low-boiling fractions of the reflux condensate to further conversion in the heating coil, together with the low-boiling fractions of the topped charging stock, subjecting motor fuel fractions of low anti-knock value recovered from the charging stock by the topping operation to reforming under independently controlled conversion 7 conditions of elevated temperature and superatmospheric pressure in.a separate heating coil, commingling the heated products from said separate heating coil with the vaporous conversion products of the process, prior to their fractionation, and subjecting said high-boiling fractions of the topped charging stock, together with said high-boiling fractions of the reflux condensate to relatively mild conversion conditions in the same cracking system without passing the same through a heating coil.

2. A process of the character defined in claim 1 wherein the high-boiling fractions of the topped charging stock and the high-boiling fractions of the reflux condensate are introduced into the reaction chamber.

3. A process of the. character defined in claim 1 wherein the high-boiling fractions of the topped charging stock and the high-boiling fractions of the reflux condensate are commingled with the stream of heated oil passing from the first mentioned heating coil to the reaction chamber.

4. A process of the character defined in claim 1 wherein the high-boiling fractions of the topped charging stock and the high-boiling fractions of the reflux condensate are commingled with the stream of heated products discharged from said separate heating coil.

5. A conversion process for producing antiknock motor fuel from hydrocarbon charging stock containing gasoline, which comprises fractionating the charging stock and separating therefrom a light fraction containing gasoline boiling hydrocarbons of inferior anti-knock value,

a heavy fraction and an intermediate fraction boiling between the light and heavy fractions, simultaneously fractionating cracked vapors, formed as hereinafter set forth, and separating therefrom relatively light and heavy reflux condensates, finally condensing the fractionated cracked vapors, heating the light reflux condensate together with said intermediate fraction to cracking temperature under pressure in a heating coil, subjecting the heavy reflux condensate and said heavy fraction of the charging stock to relatively mild conversion conditions by commingling the same with heated products discharged from said heating coil, separating the commingled oils into vapors and residue, heating said light fraction of the charging stock in a second heating coil sufficiently to enhance the anti-knock value of the gasoline hydrocarbons contained therein, and supplying resultant vapors and the vapors separated from said residue to the second-mentioned fractionating step as said cracked vapors.

6. A conversion process for prodifcing antiknock motor fuel from hydrocarbon charging stock containing-gasoline, which comprises fractionating the charging stock and separating therefrom a light fraction containing gasoline boiling hydrocarbons of inferior anti-knock value, a heavy fraction and an intermediate fraction boiling between the light and heavy fractions, simultaneously fractionating cracked vapors, formed as hereinafter set forth, and separating therefrom relatively light and heavy reflux condensates, finally condensing the fractionated cracked vapors, heating the light reflux condensate together with said intermediate fraction to cracking temperature under pressure in a heating coil, heating said light fraction of the charging stock in a second heating coil sufficiently to increase the anti-knock value of its gasoline content, subjecting the heavy reflux condensate and said heavy fraction of the charging stock to relatively mild conversion conditions by commingling the same with heated products discharged from said second coil, separating the commingled oils into vapors and residue and supplying the former to the second-mentioned fractionating step as said cracked vapors.

"7. A conversion process for producing antiknock motor fuel from hydrocarbon charging stock containing gasoline, which comprises fractionating the charging stock and separating therefrom a light fraction containing gasoline boiling hydrocarbons of inferior anti-knock value, a heavy fraction and an intermediate fraction boiling between the light and heavy fractions, simultaneously fractionating cracked vapors, formed as hereinafter set forth, and separating therefrom relatively light and heavy reflux condensates, finally condensing the fractionated cracked vapors, heating the light reflux conden sate together with said intermediate fraction to cracking temperature under pressure in a heating coil, heating said light fraction of the charging stock in a second heating coil sufficiently to increase the anti-knock value of its gasoline content, commingling the heavy reflux condensate and said heavy fract on of the charging stock with heated products from both said heating coils; separating the thus commingled oils into vapors and residue and supplying the former to the second-mentioned fractionating step as said cracked vapors.

8. A process for producing anti-knock motor fuel which comprises topping crude oil to obtain acrea e therefrom a topped crude and a light fraction containing gasoline boiling hydrocarbons of inferior anti-knock value, simultaneously fractionating cracked vapors to form a light reflux condensate and a heavy reflux condensate, sub jecting the light reflux condensate and said light fraction of the crude to cracking conditions of temperature and pressure in independent heating zones, the light crude oil fraction being heated to higher temperature than the light reflux condensate, commingling resultant hot products with the topped crude and heavy reflux con- 

